Current sensing may be used, for example, for output voltage regulation or to achieve overcurrent protection. A low-valued sense resistor may be placed in series with a current path. The current path may carry the current that is to be sensed. The current flowing through the sense resistor may produce a small voltage drop. The voltage drop may be amplified to provide a signal proportional to the current. The voltage drop across the sense resistor may, for example, be converted into a current. The sense resistor may be external to an integrated power circuit due the unavailability of integrated precision resistors that can handle high current levels.
The current levels may be reduced by a current mirror which may provide a smaller yet proportional current to the current that is to be sensed. However, the mirrored current doesn't contribute to the current flowing through the load so that the overall efficiency of the circuit may be reduced.
The current levels may also be reduced by portioning-off a small amount of current to flow through the sense resistor. The portioned-off current may be fed back to flow through the load so that no current is lost through the sensing. The voltage drop across the sense resistor may, for example be converted into a current, for example by an operational transconductance amplifier (OTA). However, the resistance of the sense resistor and the resistance of the amplifier used for voltage to current conversion (transconductance amplifier) may differ from each other by several orders of magnitude and may depend on the production process and the temperature.
Further, the on-resistance of a switching element of an integrated power circuit may depend strongly on its temperature, its production process and its overdrive voltage. This may lead to a large spread in the current that is sensed. However, it is frequently required that current sensing has to be stable and accurate over variations in temperature and supply voltage.